Electric cable and condenser insulation



United States Patent Ofificc Re. 25,188 Reissued June '19, 1962 25,188ELECTRIC CABLE AND CONDENSER INSULATION INCLUDING FISCHER-TROPSCH WAXArchibald Walter Thompson, Crayford, Kent, and Albert Sidney Freeborn,Bromley, Kent, England, assignors to Dussek Brothers & Company Limited,Crayford, England, a British company No Drawing. Original No. 2,857,350,dated Oct. 21, 1958, Ser. No. 358,978, June 1, 1953. Application for ressue Oct. 19, 1959, Ser. No. 847,442

Claims priority, application Great Britain Oct. 15, 1952 14 Claims. (Cl.26027) lVIatter enclosed in heavy brackets appears in the originalpatent but forms no part of this reissue specification; matter printedin italics indicates the additions made by reissue.

This invention is for improvements in or relating to electric cables andcondensers and has particular reference to impregnating compositions foruse in the manufacture of cables and condensers in which the dielectricconsists of fibrous material, for example paper tape, impregnated withan insulating composition.

It is known to impregnate paper tape or like fibrous materials employedfor insulating purposes with insulating oils (by which are understoodthe oils normally used either alone or admixed with other constituentsfor the impregnation of the fibrous material employed as the dielectricin the manufacture of cables and condensers) in which are incorporatedadditives, e.g. polyisobutylene, to reduce or prevent the drainage ofthe impregnant from the windings of cables which are inclined to thehorizontal. The process of impregnation must necessarily take placeabove the melting point of the impregnating compound and the cableremoved from the impregnating compound before the latter solidifies.Previously known impregnating compositions either possess relatively lowmelting points or when they have higher melting points, e.g. above 85 C.(as measured by the Institute of Petroleum Standard Method I:P.133/51,which is to be understood as used in any subsequent references tomelting point), they contain such a large amount of wax as to render thewindings brittle and to make the bending of paper-impregnated cablesdifiicult without tearing the windings. Furthermore, at impregnatingtemperatures the viscosity of the known impregnating composition is solow that serious drainage from the cable takes place after impregnationand before the saturant becomes solid and thus the impulse strength ofthe finished cable is considerably reduced.

It is an object of the present invention to provide an impregnatingcomposition the viscosity of which, at a temperature a little above itsmelting point, is sufficiently high to prevent serious drainage duringmanufacture or service and yet whose viscosity at the impregnatingtemperature, around 120 C., is not too high to prevent thoroughimpregnation.

We have now found that an insulating oil containing a proportion of asynthetic hydrocarbon wax and, if desired, a proportion ofpolyisobutylene and/ or polyethylene has a high melting point, desirableplastic characteristics at low temperatures and a sufficiently highviscosity immediately above its melting point which viscosity iscontrollable within wide limits. Such a composition may contain less waxfor the same melting point and non-drainage characteristics as knownsaturants and consequently will show reduced shrinkage on cooling andwill result in reduced void formation in the dielectric with consequentreduction of gaseous ionisation and increased cable life.

According to the present invention a composition for the impregnation offibrous material for the insulation of the conductors of cables orcondensers comprises an insulating oil and a synthetic hydrocarbon waxwith or without polyisobutylene and/or polyethylene.

The synthetic hydrocarbon wax is a wax of high melting point above C.and preferably between 85 and 120 C., and of low dielectric loss. Anexample of a suitable synthetic wax is that known under the trade nameSuper Hard Wax No. 105, which wax has a melting point of 108/1 10 C. andis a product of the Fischer Tropsch process. This wax has an averagemolecular weight of about 600 corresponding to C and its boiling pointunder vacuum is above 460 C. A further example of a suitable syntheticwax is a synthetic wax produced by the hydrogenation of less saturatednaturally occurring hydrocarbons.

The proportion of polyisobutylene and/ or polyethylene in theimpregnating composition, when employed, will depend upon the molecularweight of the polymer. Thus when low molecular weight polyisobutyleneand/ or polyethylene are used, e.g. polyisobutylene of approximatemolecular weight 1200 known as Oronite 32, or Polythene Grade 200,000 ofapproximate molecular weight 2500, a proportion of up to 40% by weighton the weight of the impregnating composition may .be used. When highmolecular weight polyisobutylene and/or polyethylene are employed, e.g.polyisobutylene of approximate molecular weight 100,000, the proportionof polymer should not, in general, exceed 1% by weight on the weight ofthe impregnating composition. The proportion of synthetic hydrocarbonwax in the impregnating composition may be up to 60% by weight andpreferably between 15% and 60% by weight on the Weight of thecomposition.

There may be be included in the composition a proportion of petrol-atum,rosin or other resins of low dielectric loss. Small proportions ofoxidation inhibitors and metal deactivators may also be incorporated inthe impregnating composition to prevent oxidation or deteriorationthereof both during manufacture and in the finished cable or condenser.

The invention includes a process for the manufacture of a compositionfor cable and condenser impregnation as above described, which processcomprises dissolving polyisobutylene and/or polyethylene in a synthetichydrocarbon wax preferably in an inert atmosphere with agitation at anelevated temperature and thereafter adding the insulating oil to themixture. Alternatively using the method of manufacture given in Britishpatent specification No. 650,088, the polyisobutylene may be dissolvedin a solvent and a mixture of insulating oil and synthetic hydrocarbonwax added thereto, the solvent then being removed.

The invention also includes cables and condensers the fibrous insulatinglayers whereof are impregnated with a composition as above described.

The following is a description by way of example oi two methods ofcarrying the invention into effect.

EXAMPLE I 350 grams of Super Hard Wax No. having a melting point of 108/110 C. are melted in an atmos phere of nitrogen in a steam-jacketedvessel and the temperature then raised to C. 32.5 grams of polyiso:butylene of molecular weight 100,000 cut into small pieces, about /2inch cubes, are then added to the molten wax with continuous stirring,the temperature of 140 C. and the atmosphere of nitrogen beingmaintained. When all the polyisobutylene has been dissolved 617.5 gramsof a cable insulating oil of low dielectric loss and viscosity 900Redwood seconds at 60 C. are added to the mixture and the whole stirreduntil homogeneous. The impregnating composition is then allowed to coolto 110 C. in the atmosphere of nitrogen and finally run )E into suitablecontainers. The final compound has a Jbbelohde drop point of around 97C.

EXAMPLE H The method of manufacture given in British patent.pecification No. 650,088 is here fol-lowed. 32.5 grams )fpolyisobutylene are cut into small pieces and dissolved n 325 gramsredistilled petroleum spirit (boiling range to 140 C.) 617.5 grams of acable insulating oil, 'iscosity 900 Redwood seconds at 60 C., are thenmixed vith 350 grams of Super Hard Wax No. 105 and held Lt 120 C., in avessel under vacuum. The polyisobutyline solution is then mixed with theinsulating oil and Wax .nd the mixture circulated at 120 C., undervacuum to he top of a tower where the mixture is broken up into lropletsand allowed to fall to the bottom of the tower. Iaturated steam at 140C. is blown in at the base of the ower and steam and solvent are carriedalong the vacum line to be subsequently condensed. The impregnat 1gcomposition collecting at the base of the tower is reirculated until allthe solvent has been removed when is passed into storage vessels.

To facilitate the removal of the petroleum spirit it is esirable toallow liquid to build up in the base of the )wer to a depth of about /3of the height of the tower nd the saturated steam is then bubbledthrough the liqid which is preferably heated by independent steam coilsontaining saturated steam at 140 C.

A typical method of impregnating a cable with the new ompositio'n is asfollows. The molten impregnating omposition, which has previously beendried and degasied is passed at 120 C. into an impregnation vesselcontining the dried paper-wound cable conductor under acuum. Theimpregnating composition is allowed to relall'l covering the cable untilimpregnation is complete hen the whole is cooled to 110 C., the vacuumbroken nd the impregnated cable removed and lead sheathed.

EXAMPLE III 150 grams of Super Hard Wax No. 105, 250 gramsalyi-sobutylene of molecular weight 1200 and 600 grams Ea cableinsulating oil of viscosity 500 Redwood seconds 60 C. are mixedtogether. The resulting impregnating )mposition had a melting point of84 C.

The dielectric properties of impregnating compositions .ade inaccordance with the present invention are good all temperatures. Theviscosity above the fusion )int of the composition is increased as thepercentage polyisobutylene is increased and decreased with ineasingsynthetic wax content.

We claim:

[1. A composition for the impregnation of fibrous aterial for theinsulation of the conductors of cables hich composition comprises acable impregnating minal oil containing between 15% and 60% by weight ofsynthetic Fischer Tropsch hydrocarbon wax having a olecular Weight ofapproximately 600, a melting point :tween 85 and 120 C. and lowdielectric loss, together ith from 1% to 40% by weight of at least onecom- )und selected from the group consisting of polyisobutyl- 1e havinga molecular weight of approximately 100,000 1d polyethylene having amolecular weight of approxiately 2500.]

[2. A composition for the impregnation of fibrous aterial for theinsulation of the conductors of cables hich composition comprises acable impregnating minal oil and from 15% to 60% by Weight of asynthetic .scher Tropsch hydrocarbon wax having a molecular eight ofapproximately 600, a melting point between i and 120 C. and lowdielectric loss, together with om 1% to 40% by weight of at least onecompound lected from the group of polyisobutylene having a mo- :ularweight of approximately 100,000 and polyethyl- [6 having a molecularweight of approximately 2500 1d petrolatum] 3. A composition for theimpregnation of fibrous material for the insulation of the conductors ofcables which composition comprises a cable impregnating mineral oil andfrom 15% to 60% by weight of a synthetic Fischer Tropsch hydrocarbon waxhaving a molecular weight of approximately 600, a melting point betweenand 120 C. and low dielectric loss, together with from 1% to 40% byweight of rosin.

[4. A composition for the impregnation of fibrous material for theinsulation of the conductors of cables which composition comprises acable impregnating mineral containing between 15% and 60% by Weight of asynthetic Fischer Tropsch hydrocarbon wax having a molecular weight ofapproximately 600, a melting point be tween 85 and 120 C. and lowdielectric loss, together with from 1% to 40% by weight of at least onecom pound selected from the group consisting of polyisobutylene having amolecular weight of approximately 100,000 and polyethylene having amolecular weight of approximately 2500 and up to 40% by weight ofpetrolatum] [5. A composition for the impregnation of fibrous materialfor the insulation of the conductors of cables which compositioncomprises a cable impregnating mineral containing between 15%and 60% byweight of a synthetic Fischer Tropsch hydrocarbon wax having a molecularweight of approximately 600, a melting point between 85 and 120 C. andlow dielectric loss, together with from 1% to 40% by weight of at leastone compound selected from the group consisting of polyisob-utylenehaving a molecular weight of approximately 100,000 and polyethylenehaving a molecular weight of approximately 2500 and up to 40% by weightof rosin] [6. A process for the manufacture of a composition for cableimpregnation which process comprises dissolving from 1% to 40% by weightof at least one compound selected from the group consisting ofpolyisobutylene having a molecular weight of approximately 100,000 andpolyethylene having a molecular weight of approximately 2500 in from 15%to 60% by weight of a synthetic Fischer Tropsch hydrocarbon wax having amolecular weight of approximately 600, a melting point between 80 and120 C. and low dielectric loss in an inert atmosphere with agitation ata temperature from to 140 C. and thereafter adding a cable impregnatingmineral oil to the mixture] [7. A process for the manufacture of acomposition for cable impregnation which process comprises dissolvingfrom 10% to 40% by weight of polyisobutylene having a molecular weightof approximately 100,000 in a solvent and adding thereto a mixture of acable impregnating mineral oil and from 15% to 60% by weight of asynthetic Fischer Tropsch hydrocarbon wax having an average molecularweight of about 600, a melting point between 85 and C. and lowdielectric loss, the solvent then being removedl 8. A composition forthe impregnation of fibrous material for the insulation of theconductors of cables which composition comprises a cable impregnatingmineral oil containing between 15% and 60% by weight of a syntheticFischer T ropsch hydrocarbon wax having a molecular weight ofapproximately 600, a melting point between 85 and 120 C. and lowdielectric loss, together with from 1% to 40% by weight of at least onecompound selected from the group consisting of polyisobutylene having amolecular weight of approximaiely 120010 100,000 and polyethylene havinga molecular weight of at least approximately 2500.

9. A composition for the impregnation of fibrous material for theinsulation of the conductors of cables which composition comprises acable impregnating mineral oil and from 15% to 60% by weight of asynthetic Fischer Tropsch hydrocarbon wax having a molecular weight ofapproximately 600, a melting point between 85 and 120 C. and lowdielectric loss, together with from 1% to 40% 75 by weight of at leastone compound sele ted from the group of polyisobutylene having amolecular weight of approximately 1200 to 100,000 and polyethylenehaving a molecular weight of at least approximately 2500 and petrolatum.

10. A composition for the impregnation of fibrous material for theinsulation of the conductors of cables which com osition comprises acable impregnating mineral oil containing between 15% and 60% by weightof a synthetic Fischer Tropsch hydrocarbon wax having a molecular weightof approximately 600, a melting point between 85 and 120 C. and lowdielectric loss, together with from 1% to 40% by weight of at least onecompound selected from the group consisting of po-lyisobutylene having amolecular weight of approximately 1200 to 100,000 and polyethylenehaving a molecular weight of at least approximately 2500 and up to 40%by weight of petrolaturn.

11. A composition for the impregnation of fibrous material for theinsulation of the conductors of cables which composition comprises acable impregnating mineral oil containing between 15% and 60% by weightof a synthetic Fischer Tropsch hydrocarbon wax having a molecular weightof approximately 600, a melting point between 85 and 120 C. and lowdielectric loss, together with from 1% to 40% by weight of at least onecompound selected from the group consisting of polyisobutylene having amolecular weight of approximately 1200 to 100,000 and polyethylenehaving a molecular weight of at least approximately 2500 and up to 40%by weight of rosin.

12. A process for the manufacture of a composition for cableimpregnation which process comprises dissolving from 1% to 40% by weightof at least one compound selected from the group consisting ofpolyisobutylene having a molecular weight of approximately 1200 to100,000 and polyethylene having a molecular weight of at leastapproximately 2500 in from 15% to 60% by weight of a synthetic FischerTropsch hydrocarbon wax having a molecular weight of approximately 600,a melting point between 85 and 120 C. and low dielectric loss in aninert atmosphere with agitation at a temperature from 110 to 140 C. andthereafter adding a cable impregnating mineral oil to the mixture.

13. A process for the manufacture of a composition for cableimpregnation which process comprises dissolving from 1% to 40% by weightof polyisobutylene having a molecular weight of approximately 1200 to100,000 in a solvent and adding thereto a mixture of a cableimpregnating mineral oil and from 15 to 60% by weight of a syntheticFischer Tropsch hydrocarbon wax having an average molecular weight ofabout 600, a melting point between 85 and 120 C. and low dielectricloss, the solvent then being removed.

14. A composition for the impregnation of fibrous material for theinsulation of the conductors of cables which composition comprises acable impregnating mineral oil containing between 15% and 60% by weightof a synthetic Fischer Tropsch hydrocarbon wax having a melting pointbetween 85 and 120 C. and low dielectric loss, together with from 1% to40% by weight of at least one compound selected from the groupconsisting of polyisobutylene having a molecular weight of approximately1200 to 100,000 and polyethylene having a molecular weight of at leastapproximately 2500.

15. A composition for the impregnation of fibrous material for theinsulation of the conductors of cables which composition comprises acable impregnating mineral oil and from 15% to 60% by weight of asynthetic Fischer Tropsch hydrocarbon wax having a melting point between6 and 120 C. and low dielectric loss, together with frot 1% to 40% byweight of at least one compound selecte from the group ofpolyisobutylene having a moleculi weight of approximately 1200 to100,000 and polyethyler having a molecular weight of at leastapproximately 256 and petrolatum.

16. A composition for the impregnation of fibrous mat. rial for theinsulation of the conductors of cables whit composition comprises acable impregnating mineral c and from 15% and 60% by weight of asynthetic Fischt T rospch hydrocarbon wax having a melting point betwei85 and 120 C, and low dielectric loss, together wi. from 1 to 40% byweight of rosin.

17. A com osition for the impregnation of fibrous mat rial for theinsulation of the conductors of cables whit composition comprises acable impregnating mineral e containing between 15% and 60% by weight ofa synthet Fischer Tropsch hydrocarbon wax having a melting poi between85 and 120 C. and low dielectric loss, togeth with from 1% to 40% byweight of at least one co-mpour selected from the group consisting ofpolyisobutylene ha ing a molecular weight of approximately 1200 to100,0( and polyethylene having a molecular weight of at least aproximately 2500 and up to 40% by weight of petrol turn.

18. A composition for the im pregnation of fibrous mat rial for theinsulation of the conductors of cables Whit composition comprises acable impregnating mineral containing between 15% and 60% by weight of asy thetic Fischer Tropsch hydrocarbon wax having a me'ltiz point between85 and 120 C. and low dielectric lo. together with from 1% to 40% byweight of at least 0. compound selected from the group consisting ofpolyis butylene having a molecular weight of approximate 1200 to 100,000and polyethylene having a molecul weight of at least approximately 2500and up to 40 by weight of rosin.

19. A process for the manufacture of a compositi for cable impregnationwhich process comprises disso. ing from 1% to 40% by weight of at leastone compou. selected from the group consisting of polyisobutylene he inga molecular weight of approximately 1200 to 100,0 and polyethylene hming a molecular weight of at let approximately 2500 in from 15 to 60% byweight 0; synthetic Fischer T ropsch hydrocarbon wax having meltingpoint between 85 and 120 C, and low dieli tric loss in an inertatmosphere with agitation at a te. perature from to 140 C. andthereafter adding cable impregnating mineral oil to the mixture.

20. A pnocess for the manufacture of a compositi for cable impregnationwhich process comprises disco ing from 1% to 40% by weight ofpolyisobutylene havi a molecular weight of approximately 1200 to 100,000a solvent and adding thereto a mixture of a cable 1'. pregnating mineraloil and from 15% to 60% by weight a synthetic Fischer Tropschhydrocarbon wax having melting oint between 85 and C. and low diell tricloss, the solvent then being removed.

References Cited in the file of this patent or the original patentUNITED STATES PATENTS 2,050,428 Diggs et a1 Aug. 11, 19 2,499,756Jacobson Mar. 7, 19 2,586,345 King Feb. 19, 19 2,594,547 Fischer Apr;29, 19 2,867,596 Bennett June 6, 1S

